This invention relates to a variable impedance circuit of the general type shown, for example, in U.S. Pat. No. 3,761,741 having an impedance which is varied through a control voltage. In particular, the invention is concerned with a variable impedance circuit in which possible generation of distortions in the impedance is suppressed to a minimum.
A variable impedance circuit providing an impedance which is variable through voltage control has found various applications. For example, such variable impedance circuit can be used for a variable highpass filter or variable attenuator in a Dolby circuit for decreasing noise upon recording and playback operations of magnetic tape recorders. In this case, the impedance is varied through a control voltage derived from the recording signal,
An important requirement imposed on the variable impedance circuit resides in that the circuit should exhibit impedance values which are not susceptible to voltages of other circuits to which the variable impedance circuit is connected, in order to have the distortions of impedance suppressed to a minimum. Further, the variable impedance circuit should provide a high impedance to the control voltage. Otherwise, current flow would be brought about due to the control voltage which would then be subjected to variation and make it impossible to obtain an impedance value proportional to the control voltage.
In hitherto known variable impedance circuits, field effect transistors or FET's have been employed so as to meet the requirements described above. However, the FET variable impedance circuits are difficult to manufacture in a semiconductor integrated circuit. There arises thus a demand for a variable impedance circuit constructed with transistor circuitries.
FIG. 1 shows an example of a hitherto known variable impedance circuit which is generally denoted by reference numeral 1 and composed of a differential current circuit 2 and a current control circuit 3.
The differential current circuitry 2 includes a first series connection of a diode 4 and an emitter-collector path of a transistor 7, and a second series connection of a diode 5 and an emitter-collector path of a transistor 6. The first and second series connection in turn are connected in parallel to each other with the transistor 6 being connected to the diode 4. The transistor 7 has a base connected to a junction between the diode 5 and the collector of the transistor 6 which is biased at the base thereof, while the transistor 7 is biased from a junction between the emitter thereof and the diode 5. The junction between the collector of the transistor 7 and the diode 4 constitutes a terminal 8 of the variable impedance circuit 1. The impedance of the circuit 1 as seen from the terminal 8 is an operating resistance of a circuit path extending from the terminal 8 through the diode 4 and the PN-junction between the base and emitter of the transistor 6 to a biasing voltage source 9, so far as the internal resistance of the current control circuitry 3 is sufficiently large. In this connection, the transistor 7 and the diode 5 serve to cause a current to flow toward a junction between the diode 4 and the emitter of the transistor 6. Accordingly, the impedance of this circuit is determined by a current flowing from a bias terminal 10 through the diode 4 and the transistor 6 to the current control circuitry 3. It is thus possible to vary the impedance by varying the current flowing through the current control circuitry 3.
As will be appreciated from the above description, there are formed two current paths in this circuit; i.e. one current path extends from the bias terminal 10 through the diode 4 to the current control circuitry 3, while the other current path extends from the bias terminal 10 through the transistor 6 to the current control circuitry 3.
Assuming now that another circuit is connected to the terminal 8 and a current is fed to the variable impedance circuit 1, the current flowing through the diode 4 will be increased, while the current flowing through the collector-emitter path of the transistor 6 will be decreased by the current flowing through the emitter-base path of the transistor 6.
On the other hand, assuming that a current is fed to a circuit connected to the terminal 8 from the variable impedance circuit, the current flowing through the diode 4 is caused to decrease, while the current flowing through the transistor 6 is increased.
Accordingly, a sum of the current flowing through the diode 4 and the current flowing through the transistor 6 will remain constant indepedently from whether current is fed to the terminal 8 or inversely fed from the terminal 8. Under such a condition, the value of impedance determined by the operating resistance of the base-emitter junction of the transistor 6 and the diode 4 will remain unvaried, as a result of which the value of impedance as seen from the terminal 8 is determined only by the current of the current control circuitry 3 on the assumption that the current control circuitry 3 has a sufficiently high impedance as compared with that of the differential current circuitry 2.
The current control circuitry 3 includes transistors 11, 12 and 13 and a resistor 14. The NPN-type transistor 11 has a collector connected to the base of the PNP-type transistor or 12, the collector of which in turn is connected to the base of the NPN-type transistor 13. With such circuit arrangement, the current i.sub.l due to a control voltage V.sub.c applied to the base of the transistor 11 is successibly amplified through the above mentioned transistors. The emitter of the transistor 12 and the collector of the transistor 13 are connected together to the differential current circuitry 2, while the emitters of the transistors 11 and 13 are grounded through the resistor 14.
Variation in the control voltage V.sub.c will bring about a corresponding variation in the current amplified through the transistors 11, 12 and 13, which in turn results in variation in the current flowing through the differential current circuitry 2, involving variation in impedance.
It should be noted herein that, in the case of the above described variable impedance circuit, the impedance of the current control circuitry 3 can not be sufficiently high relative to that of the differential current circuitry 2, because the emitter of transistor 12 is connected to the differential current circuitry 2. Thus, the current fed externally to the terminal 8 will flow in the transistor 12 thereby disadvantageously degrading the differential operation of the differential current circuitry 2.
Additionaly, the control voltage V.sub.c is required to vary in a voltage range in which the transistor 12 is not driven into saturation. Due to such a requirement in combination with the fact that the emitter voltage of the transistor 12 is determined by the potential at a junction between the diode 4 and the emitter of the transistor 6, the range in which the control voltage is allowed to vary is undesirably limited by the above potential.